In the process of exploration and exploitation of mineral deposits, notably gold and platinum group metal deposits, ore sampling is a specifically difficult problem because such elements in nature are distributed extremely irregularly and their respective quantities are very small. Hence, an analytical method is required to be capable of determining such elements in sufficiently large samples with a high sensitivity.
Fire assay has been the industry standard for the majority of mineral deposit evaluations and has been the method of choice for gold analysis. However the procedure requires complicated sample preparation, is very labour intensive as most steps in the method occur in crucibles and cupels and involves extremely high temperatures (˜1100° C.). Furthermore, the analysis is typically performed in an offsite laboratory. Rapid results by this technique are therefore not possible. Furthermore, the small mass of sample analysed, typically 20-50 g, can introduce significant sampling errors for inhomogeneous ore samples. Furthermore, the original sample is destroyed in the process, preventing subsequent reanalysis.
An alternative method for the analysis of elements in mineral ores, including gold, is the gamma-activation analysis method (GAA), GAA is based on sample activation by highly energetic gamma-rays. The GAA method involves irradiating samples with an accelerator producing high energy Bremsstrahlung X-rays.
For the analysis of gold, X-rays having an end-point energy of around 8 MeV are optimal. These will activate any gold in the sample and the activated gold-nuclei decay to produce a 279 keV gamma-ray. A detector then counts the gamma-rays produced. For the measurement of the platinum group metals and many other elements, a higher X-ray end-point energy is required, typically in the range 11-14 MeV. Each element produces one or more gamma-rays of characteristic energy by which it may be identified and quantified.
There is a direct relationship between the gamma-ray strength and the quantity of the target element, which allows the elemental content of the sample to be determined. However, to accurately determine the elemental content of the sample requires accurate knowledge of the intensity and the energy spectrum of the X-ray source, which is highly susceptible to factors such as temperature variations within the accelerator.
For example, variations of a few hundred keV about a nominal X-ray end-point energy of 8 MeV can change the activation yield for gold by tens of percent. If uncorrected, this would lead to correspondingly large errors in the estimated gold content of the samples.
Throughout this specification the word “comprise”, or variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.
Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present disclosure as it existed before the priority date of each claim of this application.